Estimation of Human Ankle Impedance During the Stance Phase of Walking

Abstract

Human joint impedance is the dynamic relationship between the differential change in the position of a perturbed joint and the corresponding response torque; it is a fundamental property that governs how humans interact with their environments. It is critical to characterize ankle impedance during the stance phase of walking to elucidate how ankle impedance is regulated during locomotion, as well as provide the foundation for future development of natural, biomimetic powered prostheses and their control systems. In this study, ankle impedance was estimated using a model consisting of stiffness, damping and inertia. Ankle torque was well described by the model, accounting for 98 ± 1.2% of the variance. When averaged across subjects, the stiffness component of impedance was found to increase linearly from 1.5 Nm/rad/kg to 6.5 Nm/rad/kg between 20% and 70% of stance phase. The damping component was found to be statistically greater than zero only for the estimate at 70% of stance phase, with a value of 0.03 Nms/rad/kg. The slope of the ankle’s torque-angle curve—known as the quasi-stiffness—was not statistically different from the ankle stiffness values, and showed remarkable similarity. Finally, using the estimated impedance, the specifications for a biomimetic powered ankle prosthesis were introduced that would accurately emulate human ankle impedance during locomotion.

Design and characterization of a biologically inspired quasi-passive prosthetic ankle-foot

“…The measured stiffness of the pneumatic ankle prosthesis was compared to the stiffness of the ESR prosthetic foot, and recently published biological ankle stiffness values [5]. To fully understand how the pneumatic ankle prosthesis is different than the conventional ESR prosthetic foot, the stiffness data were compared as a function of ankle angle and normalized COP location (Fig.…”

“…Corrspondingly, for the ESR foot, stiffness is only a function of COP location, and movment along the horizontal axis does not affect stiffness magnitude. Human data reported from Rouse et al [5], shown for a 70 kg individual.…”

“…This opposes the trend known to occur in the biological ankle joint. The stiffness of the biological ankle joint is known to increase linearly during the dorsiflexion region of stance phase [5]. Therefore, the anthropomorphic cantilever design of ESR feet results in non-biological mechanical behavior, likely contributing to the gait deficits of transtibial amputees.…”

A Cyber Expert System for Auto-Tuning Powered Prosthesis Impedance Control Parameters

“…Biological joint impedances have been computed directly from experimental measurements and estimated using biomechanical models. 14,24,[31][32][33][34]42,43 Due to experimental limitations, in vivo joint impedance during ambulation has only been measured at the ankle during the stance portion of gait. 31 Impedance measurements at other joints, such as the knee, were made under static or quasi-static conditions and therefore may not transfer to dynamic ambulation tasks.…”

“…14,24,[31][32][33][34]42,43 Due to experimental limitations, in vivo joint impedance during ambulation has only been measured at the ankle during the stance portion of gait. 31 Impedance measurements at other joints, such as the knee, were made under static or quasi-static conditions and therefore may not transfer to dynamic ambulation tasks. 14,24,42,43 Impedance estimated from musculoskeletal biomechanical models has only been validated for the stance phase of gait.…”